Currently, and in the past, progressive cavity pumps have been ran in two pieces. First, the stator portion is ran in on standard jointed tubing. Then, the rotor portion is ran in on either jointed rods, or co-rod and stabbed into the stator. The rods are then connected to a rotary head at surface which turns the entire rod string and subsequently the rotor, which is inside the stator, and thus creating the pumping action. This type of system has a large number of disadvantages. The entire process requires multiple pieces of equipment, service rig, rod rig, co-rod rig, accelerators, tubing x-ray inspectors, etc. which leads to high service times and large man power exposure. Due to the nature of the pumping system it also requires various down hole and surface tools, such as stuffing boxes, no turn tools, tubing rotators, rotary heads, tag bars, etc.
One of the main disadvantages of this system is the mechanical wear that occurs on the rod and tubing string due to the rotation of the rods. This usually eventually wears holes in the jointed tubing, and weakens the rods. This leads to rod/tubing failures, which then require servicing. Additionally, because the rods are rotated from surface, when a pump seizes or fails, the rotary head at surface builds up and stores torque. This creates the necessity to run additional tools such as an anti-rotational tool. This is ran, because when the torque is let off of the rod string the string tends to back turn violently (which besides being a safety concern) can cause the tubing to back off and come apart, thus falling down hole. This highlights another limitation of this system in that you cannot turn the rotor backwards (which would be advantageous) because the tubing may back off, or any of the rod connections may back off because when rotating backwards, the threads can loosen off.
The rod/tubing combination is also a limitation because the rods are ran inside the production tubing which then takes up space and causes additional restriction of the production area. The rods also increase the overall surface area, which increases friction loss. Additionally the friction loss is difficult to combat because of the concentric nature of this design. Alternative materials (plastics, fiberglass, etc.) that would normally assist in friction reduction cannot be used due to the aggressive nature of the rotation of the steel rods.
It is also difficult to space out the rotor properly. Spacing out, is when the rotor and rods are ran into the well and the rotor is stabbed into the stator, it is necessary to land it in an appropriate place so that as the rod string stretches due to string weight and other factors, the lobes line up with the cavities. To do this a tag bar is normally ran on the bottom of the stator. This allows the rig crew to lower the rotor until it tags the tag bar. Then measurements are used to pull up to a certain spot and hang the rod string. This action, while fairly reliable, is by no means certain.
The current method of application of rods and tubing is all steel. This is a major drawback, as these types of wells tend to have a variety of corrosive fluids and gases present. This very often leads to corrosion issues on the production string and rods, as well as scale build up in the production string and rods. It would be very advantageous to use plastic lined products as the production conduit for corrosion/scaling protection, as well as friction reduction. Due to the rotary action of the rods, lined jointed tubing cannot be used as the rods would beat it up, and destroy it with their rotary motion and wear.
The conventional system of rod strings extending through the production string allows for a multiple unit service called a flush. Often with heavy oil wells, the pump sands off and this requires servicing. To do this, often instead of pulling the entire completion, a coiled tubing unit with small coil is brought to location, where it then runs in beside the rods and tubing, down to the top of the rotor where the tubing string is then circulated clean. The coil unit then pulls out of the well, and a flush-by unit is used to pull the entire rod string up which is connected at the bottom to the rotor. This action pulls the rotor out of the stator. The flush-by then begins to inject water or oil into the production string forcing the through the stator into the well bore, forcing the well onto a vacuum. Once a certain amount of fluid has been pushed into the formation, the rotor is lowered back into place, the rod string is re-hung, and standard pumping operations begin. This operation also requires multiple service units, and often, because of the unpredictability of the rods inside of the production tubing, the coil unit may not be able to get entirely down, or worse, could become stuck, or lodged around the rods. Basically, things start to get pretty congested with rods and coiled tubing inside of small diameter, normally 3.5′ O.D., production tubing.
When a flush is preformed, the fluid that is pushed/flushed down into the well bore mixes with any solids in the hole, and helps to suspend the solids for a time so that when you put the pump back on normal operations, the mix of fluids and solids can be pumped to surface as per normal. In order to perform the aforementioned flush, currently it is necessary to remove the rotor from the stator so that one can flush down through the stator into the well bore with a fluid pump at surface. Once this is achieved, the rotor is then lowered back into the stator, and normal pumping operations can resume.
This moving of the rotor up and down is usually accomplished with the above mentioned flush-by unit, or a service rig, both of which normally have the fluid pump with them. It is time consuming, and typically does not occur until the rotor has already torqued up due to solids as the only way to diagnose this prior to torquing up is with logic programming. Unfortunately if the programming reads it is torquing up, all it can do is shut it down. The system then sits static until equipment can be mobilized, (which can be days) and while the well sits idle, the solids that are suspended in the production column begin to settle back down on top of the rotor, which typically means that when the equipment arrives, the flush-by cannot pull the rotor out of the stator to perform the flush. The well then also requires a coiled tubing unit to clean out on top of the rotor before the flush can begin. If the coil unit is unsuccessful, a complete service may be required with a service rig which includes pulling everything out of the hole, including the tubing.
In current configurations, the progressive cavity pump (PCP) is deployed on standard tubing and rods (or co-rod). As mentioned above, the connections that are inherent with this type of system are prone to backing off if the rods/pump are turned backwards. Additionally, as the rods torque up, they store energy, so that once the system goes down on high torque, the rods have a lot of stored energy. To release that energy, the rotary heads at surface are turned backwards, or the hydraulic pressure is allowed to bleed of, which allows the torque in the rods to dissipate by back spinning, sometimes very violently. When this occurs, there is a risk of the aforementioned back off of the tubing.
If that occurs, the tubing/rods can fall down the hole, causing additional problems. Currently, to combat this backing off of the tubing, an external no-turn tool is commonly ran. It is connected towards the bottom of the tubing string, and contacts the casing of the well, and stops the tubing from turning backwards in the event of the rods spinning backward. It does not stop the rods from backing off as mentioned above, as the rods are inside the tubing, and the no-turn tool operates only on the jointed tubing. Because this tool is in contact with the casing, it is difficult, or impossible to get past it with anything to clean out the cellar/sump of the well. This means that over time, as the sump fills up with solids, the only way to clean it out, is to pull everything out of the hole, and perform a comprehensive cleanout. Flushes only flush to the intake of the pump, and do not clean the sump/cellar so periodic cleanouts are still necessary.